Unlike this guy, Mike's post:
"Additionally, you can simply wash the lead off when you clean the gun. With a piece of leather, you have to remove it from the gun or the powder residue that gets impregnated into the leather can cause the cock jaws to rust like crazy. Mike Irwin, Jan 18, 2003"
I've never had any rusting of the cock's jaws. Don't know why, as I scrub the lock, cock, jaws, fling and leather or lead with a water soaked 'old' toothbrush.
THEN I wipe and blow off the lock with compressed air (usually), then spray copiously (FLUSHING) with WD40. Perhaps THAT is the secret for not rusting, perhaps the WD40 'chases' the water out of the leather, replacing it with the oil? Who knows, but I do not get "rust like crazy" with either lead or leather.